Field of the Invention
This invention relates to system and method for producing carbonized biomass using a twin-screw extruder.
Description of Related Art
The Hydrothermal Carbonization (HTC) of biomass, primarily lignocellulosic biomass, to produce a densified carbon product (biochar) is known in the art. Presently, HTC is most often carried out with mixtures of biomass and water in a sealed batch reactor (typically a Parr Reactor, maintained at 200-300° C. for 0.1 to 1 hour) of the sort invented by Friedrich Bergius who first developed the tools and techniques required to study the reactive chemistry of compounds subjected to high pressures and temperatures. Dr. Bergius was awarded a Nobel Prize for his work in high-pressure chemistry in 1931.
Because HTC biomass resembles coal, one early use of HTC technology was to produce a coal-like material from the different constituents of lignocellulosic biomass and then attempt to relate the chemical and mineral characteristics of the HTC biomass product to the components of coal as known through petrographic analysis. The HTC process tends to remove oxygen and some hydrogen from biomass and produce a material that is similar to a low-rank coal. FIG. 1 shows a diagram first developed by Van Krevelen which reveals the difference between Loblolly pine biomass and typical US coals as a function of the ratio of atomic hydrogen to carbon when graphed as a function of the atomic ratio of oxygen to carbon. This figure suggests that HTC pine biomass is chemically similar to low-rank coal.
FIG. 2 shows a similar diagram that includes a number of measurements for raw lignocellulosic biomass and similar biomass that has been mixed with water and subjected to HTC in batch Parr reactors to produce a biochar.
In this study, three woody and three herbaceous biomass feedstocks were subjected to HTC in a Parr reactor. Each feedstock was treated in liquid water for 30-minutes at temperatures ranging from 175° C. to 295° C. Gaseous, aqueous, and solid hydrochar products were characterized to examine the effects of process temperature upon product yields, compositions, and energy densification. With increasing temperature, the mass of solid hydrochar product was reduced, and its energy density increased. At temperatures ≥255° C., hydrochars produced from the woody feedstocks had energy contents of 28-30 MJ/kg, comparable to sub-bituminous coal. Hydrochars from the herbaceous feedstocks had lower energy contents, but still showed 20-40% energy densification compared to raw materials. Energy densification by the HTC process involves preferential loss of oxygen. With increasing process temperature, the atomic O/C ratio was reduced from 0.6-0.7 in raw feedstocks to approximately 0.2 for all hydrochars produced at ≥275° C. Yields of gaseous products (consisting predominantly of CO2) increased with increasing HTC temperature, reaching 10-12% at temperatures ≥275° C. The sum of sugar and organic acid yields in the aqueous product fraction was typically 8-12%, at process temperatures ≥215° C., although the detailed composition of these products varied considerably with temperature. Water is also produced by the HTC of biomass. One of the inventors has reported water production yields ranging from 4-20%, as a percentage of starting dry mass, for a variety of biomasses subjected to HTC at a processing temperature of 255° C.